Since fuel economies and the need to protect the environment have become priorities, it has become desirable to produce elastomers with good mechanical properties and as low a hysteresis as possible so that they can be used in the form of rubber compositions usable for the manufacture of various semi-finished products involved in the constitution of tires, such as, for example, underlayers, calendering or sidewall rubbers, or treads, and to obtain tires with improved properties, having in particular reduced rolling resistance.
To achieve such an objective, numerous solutions have been proposed, first of all ones essentially concentrating on the use of elastomers modified by means of agents such as coupling, starring or functionalizing agents, with carbon black as the reinforcing filler, with the aim of obtaining a good interaction between the modified elastomer and the carbon black. It is generally known that in order to obtain the optimum reinforcement properties imparted by a filler, the latter should be present in the elastomeric matrix in a final form which is both as finely divided as possible and distributed as homogeneously as possible. Now, such conditions may only be obtained insofar as the filler has a very good ability firstly to be incorporated into the matrix during mixing with the elastomer and to disagglomerate, and secondly to be dispersed homogeneously in this matrix.
It is known that carbon black has such abilities, which is generally not true of white or inorganic fillers. For reasons of mutual attraction, the inorganic filler particles have a tendency to agglomerate together within the elastomeric matrix. These interactions have the harmful consequence of limiting the dispersibility of the filler and hence the reinforcing properties to a substantially lower level than that which it would be theoretically possible to achieve if all the (inorganic filler/elastomer) bonds that could be created during the mixing operation were in fact obtained. These interactions also tend to increase the consistency of the rubber compositions and therefore to make them more difficult to work (“processability”) in the uncured state than in the presence of carbon black.
However, interest in rubber compositions reinforced with inorganic filler was greatly revived with the publication of European Patent Application EP 0 501 227, which discloses a sulfur-vulcanizable diene rubber composition, reinforced with a special precipitated silica of the highly dispersible type, which makes it possible to manufacture a tire or tread with substantially improved rolling resistance, without adversely affecting the other properties, in particular those of grip, endurance and wear resistance. Patent Applications EP 0 810 258 and WO99/28376 disclose diene rubber compositions reinforced with other special inorganic fillers, specifically aluminas or aluminum (oxide-)hydroxides, of high dispersibility, which also make it possible to obtain tires or treads having such an excellent compromise of contradictory properties.
Although the use of these specific, highly reinforcing, siliceous or aluminous inorganic fillers has reduced the difficulties of processing the rubber compositions that contain them, such rubber compositions are nevertheless more difficult to process than rubber compositions filled conventionally with carbon black.
In particular, it is necessary to use a coupling agent, also known as a bonding agent, the function of which is to provide the connection between the surface of the inorganic filler particles and the elastomer, while facilitating the dispersion of this inorganic filler within the elastomeric matrix.
The term “coupling agent” (inorganic filler/elastomer) is understood to mean an agent capable of establishing a sufficient chemical and/or physical connection between the inorganic filler and the elastomer. Such a coupling agent, which is at least bifunctional, has, for example, the simplified general formula “Y-T-X”, in which:                Y represents a functional group (“Y function”) which is capable of bonding physically and/or chemically with the inorganic filler, where the bond is able to be established, for example, between a silicon atom of the coupling agent and the hydroxyl (OH) surface groups of the inorganic filler (for example, surface silanols in the case of silica);        X represents a functional group (“X function”) which is capable of bonding physically and/or chemically with the elastomer, for example by means of a sulfur atom; and        T represents a hydrocarbon group making it possible to link Y and X.        
The coupling agents must not be confused with simple agents for covering the inorganic filler which, as is known, may comprise the Y function which is active with respect to the inorganic filler but are devoid of the X function which is active with respect to the elastomer.
Silica/elastomer coupling agents in particular have been described in a large number of documents, the best known being bifunctional alkoxysilanes. For example, in French patent application FR 2 094 859, it was proposed to use a mercaptosilane for the manufacture of tire treads. It was quickly shown, and is today well known, that the mercaptosilanes, and in particular γ-mercaptopropyltrimethoxysilane or γ-mercaptopropyltriethoxysilane, are capable of giving excellent silica/elastomer coupling properties, but that these coupling agents cannot be used industrially because of the high reactivity of the —SH functions. Specifically, this high reactivity rapidly results in premature vulcanization or “scorching” during the preparation of the rubber composition in an internal mixer, in very high Mooney plasticity values, and in rubber compositions which are virtually impossible to work and process industrially. To illustrate this impossibility of using such coupling agents bearing —SH functions and the rubber compositions that contain them industrially, documents FR 2 206 330 and U.S. Pat. No. 4,002,594 may be cited, for example.
To overcome this drawback, it has been proposed to replace these mercaptosilanes by polysulfurized alkoxysilanes, in particular bis-(C1-C4)alkoxysilylpropyl polysulfides such as described in numerous patents or patent applications. (See, for example, FR 2 206 330 or U.S. Pat. Nos. 3,842,111; 3,873,489; 3,978,103; and 3,997,581.) These polysulfurized alkoxysilanes are generally considered to be the products which, for vulcanized rubber compositions filled with silica, give the best compromise in terms of resistance to scorching, processability and reinforcing power. These polysulfides include, for example, bis-3-triethoxysilylpropyl disulfide (abbreviated to TESPD), more particularly bis-3-triethoxysilylpropyl tetrasulfide (abbreviated to TESPT), which is known as an (inorganic filler/diene elastomer) coupling agent which is effective (and frequently used) in rubber compositions for tires, in particular those intended to form treads. TESPT is sold, for example, by Degussa under the name “Si69”.